1. Field of the Invention
The present invention relates to an apparatus and method for improving the efficiency of a Power Amplifier (PA). More particularly, the present invention relates to an apparatus and method for improving the efficiency of a Doherty PA by switching the supply of a peaking amplifier (e.g., peaking transistor) of the Doherty PA.
2. Description of the Related Art
As portable terminals continue to gain popularity, manufacturers continue to seek new ways in which to improve them. One aspect to improve a portable terminal is to reduce its size and weight, thus making it more convenient to carry. Another aspect is to provide advanced features. However, reductions in size and weight are often limited by the power requirements necessary to provide the advanced features. That is, to supply the necessary power required for processing and supporting of the advanced features, the battery of the portable terminal must be adequately sized to ensure sufficient capacity for a reasonable amount of time. Thus, a reduction in the size of the portable terminal, due to the necessary size of the battery, is difficult to obtain.
Another consideration in battery sizing, and in power use in general, is the amount of power necessary to transmit a wireless signal. For transmission of the wireless signal, the portable terminal uses a Radio Frequency (RF) transmitter that includes a Power Amplifier (PA). The PA is used to amplify signals for outputting by the portable terminal's antenna. If the PA is operated inefficiently, additional power is necessary to transmit a signal. Accordingly, there is a great amount of research directed towards improving the performance and efficiency of a PA.
Techniques for improving the efficiency of a PA can be broadly classified into the categories of 1) supply modulation, 2) load modulation (i.e., a Doherty PA), 3) out-phasing, and 4) Pulse Width and Pulse Position Modulation (PWPPM). Out of these four techniques, PWPPM is not deemed suitable for a signal having a wide-bandwidth or a signal having a high Peak to Average Power Ratio (PAPR) such as a signal modulated in a communication system using Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA). Also, although out-phasing techniques achieve very good drain efficiency, they waste considerable power in a power combiner of the PA. Thus, supply modulation and load modulation are the prevalent techniques that are used in industry to achieve high PA efficiency.
Supply modulation is a very effective technique to achieve very high drain efficiency of a PA. For example, peak drain efficiencies of 70-80% have been achieved using supply modulation techniques. Accordingly, most research effort, in both industry and academia, regarding improving the efficiency of a PA is directed towards supply modulation techniques. Supply modulation techniques include Envelope Tracking (ET) techniques for use with a linear amplifier, and Envelope Elimination and Restoration (EER) techniques for use with a saturated switching amplifier. The EER technique is known to achieve higher efficiency than the ET technique. However, it suffers from a number of practical implementation drawbacks. These drawbacks include low gain, stringent timing alignment requirements, high supply modulation bandwidth requirements, high phase modulation path bandwidth requirements, a low Power Added Efficiency (PAE) and direct feed-through contamination at low output, and a low Power Supply Rejection Ratio (PSRR). For these reasons, ET is often the preferred technology as compared to EER.
In a communication system having a wide-bandwidth signal and using an OFDM/OFDMA scheme, such as a Long Term Evolution (LTE) system or a Worldwide Interoperability for Microwave Access (WiMAX) system, the PAPR of the transmitted signal is high, which creates difficulty in achieving high efficiency. To address this problem, supply modulation techniques apply WideBand Envelope Tracking (WBET) and sophisticated digital pre-distortion and crest factor reduction to achieve an average drain efficiency of up to 70% while maintaining a relatively moderate gain (about 10-12 dB) as well as linearity. However, there occurs a bottleneck in the efficiency of such a supply modulator. That is, despite significant research efforts and specially designed modulators, the reported results shows that the average efficiency of the modulator is restricted to 70-75% when the supply is modulated according to the envelope of an LTE/WiMAX signal. Consequently, the final-stage PA efficiency is restricted to 40-60%, even in the best case scenario since the modulator efficiency multiplied by the transistor drain efficiency equals the final-stage PA efficiency (e.g. 70%*70%=49%).
A Doherty amplifier that employs a load modulation technique includes two RF amplifiers. The first is a carrier transistor and the second is a peaking transistor. Currently, state of the art Doherty amplifiers can achieve an average final-stage efficiency of 40-60% when used in an LTE/WiMAX type system using load modulation without any kind of supply modulation. To improve on this efficiency, two techniques have been introduced. The first is modulation of the gate bias of the peaking transistor and the second is the modulation of the supply of the Doherty transistor.
There has been extensive research regarding gate bias modulation of the peaking transistor in a Doherty PA. However, gate bias modulation is typically aimed at achieving a better linearity near the ‘switch on’ signal level of the peaking transistor and does not specifically target efficiency improvement. Moreover, even if gate bias modulation is employed for efficiency improvement, the characteristics of a Doherty PA are such that gate bias modulation can only provide a 1-2% efficiency improvement.
On the other hand, there has not been nearly the amount of research performed on supply modulation in a Doherty PA. One known approach was directed towards modulating the supply of the carrier transistor (or carrier amplifier) in the Doherty PA since it is always on and it typically delivers more average power than peaking transistor (or peaking amplifier). However, based on the already present load modulation, the carrier transistor already operates near a 1 dB compression region at higher power. Hence, improvement of drain efficiency using a supply modulation technique at the carrier transistor is limited. Also, if the supply of the carrier transistor is modulated, the efficiency of the carrier transistor is multiplied by the efficiency of the supply modulator. Thus, the benefits of supply modulation for the carrier transistor are further limited. Moreover, the costs of modulating the supply of the carrier transistor are greater than those when using a simple linear PA with WBET, but the efficiency gains are substantially the same. As another alternative, two different supply modulators may be used for the carrier transistor and peaking transistor which may achieve better efficiency. However, the system costs would increase significantly. Accordingly there is a need for an apparatus and method for improving the efficiency of a Doherty PA.